1. Field of the Invention
The present invention relates to an optical information recording medium in which a pre-pit longer than the diameter of a reproducing radiation spot and a pre-pit shorter than the diameter of the reproducing radiation spot coexist in the same track, especially, the same sector, and also relates to a method of fabricating such an optical information recording medium.
2. Description of Related Art
There is known a write-once type or rewritable optical information recording medium in which a train of pre-pits and a pre-groove are predominantly formed in a recording region and a laser beam is used as a reproducing radiation spot. The train of pre-pits and pre-groove are formed with a concave/convex form in the surface of a substrate. A signal is optically read out by irradiating the substrate with the reproducing radiation spot having a diameter larger than the width of each of the pre-pits and pre-grooves to detect the intensity of light reflected from the substrate or light transmitted through the substrate by an optical detector. Namely, at a portion where the pre-pit is present, the intensity of light incident upon the optical detector decreases since the interference and diffraction of light are produced between light with which the pre-pit itself is irradiated and light with which a land portion having no pre-pit and pre-groove formed thereon is irradiated. On the other hand, at a portion where no pre-pit is present, the intensity of light incident upon the optical detector increases since such a phenomenon of interference and diffraction of light is not produced. Accordingly, a pre-pit signal can be read on the basis of the waveform of an output signal of the optical detector.
In order to facilitate the reading of a signal, it is preferable that the level of a production signal or a difference in intensity of light upon the optical detector between the portion including the pre-pit and the portion including no pre-pit is large.
The intensity of light incident upon the optical detector from the portion including the pre-pit depends on the size of the pre-pit. Provided that the refractive index of the substrate is n.sub.1, the refractive index of the air is n.sub.2, the wavelength of the reproducing radiation is .lambda. and the diameter of the reproducing radiation spot is .phi., the intensity of light incident upon the optical detector in the case of a reflection type of optical information recording medium becomes the minimum to allow correct detection of the pre-pit when the depth (or height) d of the pre-pit is equal to N.multidot..lambda./4n.sub.1 (N: is a positive odd number) and the width of the pre-pit is equal to .phi./3. In the case of a transmission type of optical information recording medium, the intensity of reflected light incident upon the optical detector becomes minimum when the depth (or height) d of the pre-pit is equal to N.multidot..lambda./4(n.sub.1 -n.sub.2) and the width of the pre-pit is equal to .phi./3. See pages 29 and 30 of "SYNTHETIC COLLECTION OF OPTICAL MEMORY AND OPTO-MAGNETIC MEMORY TECHNOLOGIES" published on Oct. 31, 1983 by Science Foram Co., Ltd.
The above-mentioned type of optical information recording medium is fabricated in such a manner that a prototype or master having a photoresist layer formed thereon is irradiated with a signal-modulated cutting light to cut or form grooves serving as the origins of the above-mentioned pre-pits and pregrooves, and an inverse pattern of the cut grooves is transferred from the prototype to a substrate.
The width of the groove (pre-pit) cut in the photoresist layer is proportional to the product of the intensity of the cutting light and the irradiation time thereof. Accordingly, when a longer pre-pit and a shorter pre-pit are cut by use of cutting light, the intensity of which is fixed, the width of the longer pre-pit with a longer pit length requiring a longer irradiation time of the cutting light becomes large while the width of the shorter pre-pit becomes small. Therefore, if the width of either one of the longer pre-pit and the shorter pre-pit is selected to have the above-mentioned optimum value, the width of the other pre-pit will take an undesirable value deviated from the optimum value, thereby affecting correct recording or reproduction of information. Especially, when the intensity of the cutting light is set such that the width of the longer pre-pit takes the optimum value, the level of a reproduction output from the shorter pre-pit cut by the cutting light having such an intensity becomes remarkably low and hence it is not possible to cope with a demand for improvement of a recording density by shortening each of the longer pre-pit and the shorter pre-pit.
In order to eliminate the above-mentioned inconvenience, the assignee of the present application has proposed an optical information recording disk characterized in that the width of a pit shorter than the diameter of a reproducing laser spot is made larger than that of a pit longer than the diameter of the reproducing laser spot (JP-A-61-214149). The assignee of the present application has also proposed a method of fabricating such an optical information recording disk. In the proposed method, a prototype or master optical disk is driven to rotate at a constant angular velocity. In an outer region of the prototype optical disk where the pit length of the smallest or shortest pit is longer than the diameter of the reproducing laser spot, a photosensitive surface of the prototype optical disk is exposed with an exposure intensity which is proportional to a distance from the center of rotation of the prototype optical disk to the center of a cutting laser spot located at the outer region of the prototype optical disk. On the other hand, in an inner region of the prototype optical disk where the pit length of the smallest or shortest pit is shorter than the diameter of the reproducing laser spot, the photosensitive surface of the prototype optical disk is exposed with the amount of exposure which is equal to the amount of exposure for pits in the region where the pit length is longer than the diameter of the reproducing laser spot. According to this method, the pre-pit having a shorter pit length is formed with a wider width and hence the level of a reproduction output signal is prevented from being lowered or deteriorated. Therefore, the signal length of a recording signal (or the pit length corresponding to the recording signal) can be shortened. As a result, it is possible to make the recording density high, to make the recording capacity large and to make the size of an optical information recording disk small.
The above-mentioned optimum value for pre-pit width is substantially true for an in-groove recording type of optical information recording medium or an optical information recording medium which has no pre-groove and in which a track pitch is adjusted such that a reproducing radiation spot does not extend to or reach a track adjacent to a track from which a signal is to be reproduced.
However, especially, in an optical information recording medium such as an on-land recording type of optical information recording medium or an optical information recording medium having a relatively narrow track pitch in which the peripheral portion of a reproducing radiation spot extends to or cover a track adjacent to a track from which a signal is to be reproduced, if the width W of a pre-pit is selected to take the above-mentioned optimum value, the amount of light impinging upon a land portion is decreased by a portion of the reproducing radiation spot extending to and covering the adjacent track, thereby reducing the effect of interference of light to increase the intensity of light incident upon an optical detector. As a result, the level of a production output from a pre-pit shorter than the diameter of the reproducing radiation spot is deteriorated while a pre-pit longer than the diameter of the reproducing radiation spot yields a reproduction signal waveform 41 which involves a distortion 42, as shown in FIG. 1. The distortion component 42 is produced because the amount of light reflected from the side wall portions of the pre-pit longer than the diameter of the reproducing radiation spot is increased when the reproducing radiation spot lies in a central portion and does not cover the front and rear portions of the pre-pit in a longitudinal direction thereof. If a signal distortion component indicated by dotted line 42 in FIG. 1 is superimposed on a normal detection waveform indicated by solid line 41, there is a possibility that the processing of the reproduction signal erroneously detects a waveform, as indicated by one-dotted chain line in FIG. 1, which has a level L lower than the correct or normal detection level.
Pre-pits having longer pit lengths and pre-pits having shorter pit lengths coexist in the pre-pit train, as has been mentioned above. In the case when the width of the pre-pit longer than the diameter of the reproducing radiation spot is formed with the above-mentioned optimum value, there is a problem that the flow of a resin upon injection shaping of a substrate is arrested, thereby deteriorating the transferability of pre-pits.
In order to eliminate such inconveniences, there has been proposed an optical information recording medium in which the front end portion 43a and rear end portion 43b of each pre-pit 43 are tapered, as shown in FIG. 2. When the pre-pit 43 is formed as shown in FIG. 2, any distortion of the waveform can be effectively eliminated but the level L of a reproduction signal is deteriorated, as shown by one-dotted chain line in FIG. 1, so that an error is liable to be generated upon reading of a signal.
The above-mentioned method of fabrication of an optical information recording disk which has been proposed by the assignee of the present application has a drawback, as will be explained just below, in the case where a recording region configured into a ring shape is partitioned in its circumferential direction into a multiplicity of sectors and a sector mark pit representative of the partition for each sector and ID pits inclusive of address pits and synchronizing pits are preliminarily formed for each sector.
Namely, the sector mark pit is elongated as compared with the ID pit in order to discriminate them from each other. For example, when each of the time length of an address signal and the period of a synchronizing signal read from an optical information recording disk rotationally driven at 1,800 r.p.m. is 90 ns, the sector mark is formed with a length of 540 ns or 900 ns. In the case where each pre-pit is formed with such a length, the sector mark becomes longer than the diameter of the reproducing radiation spot even in a region where the ID pit is shorter than the diameter of the reproducing radiation spot, or pits shorter than the diameter of the reproducing radiation spot and pits longer than the diameter of the reproducing radiation spot coexist in the same sector of the same track.
If the above-mentioned conventional method is applied to such a pattern of pre-pits, that is, if all of the pre-pits included in the same track are exposed with the same power, the sector mark portion having a longer length is subjected to excessive exposure since the size or width of a pre-pit formed is proportional to the product of the power of a cutting radiation beam and the irradiation time thereof, as has already been mentioned. .As a result, the width of the sector mark becomes wider as compared with that of the ID pit.
Though in the foregoing explanation the sector mark and the ID pit have been exemplified as a pit longer than the diameter of the reproducing radiation spot and a pit shorter than the diameter of the reproducing radiation spot, respectively, the above-mentioned problem may arise irrespective of the kind of pre-pits in any case where a pit longer than the diameter of the reproducing radiation spot and a pit shorter than the diameter of the radiation spot coexist.